Users of mobile units such as computers benefit from having access to State of Charge (SOC) information about the battery powering the device, during both the discharging and charging phases of operation. The chief benefits come from knowing what portion of the battery energy remains during discharge and how much remains to be charged during charging. The generation of this SOC information is traditionally facilitated by a “coulomb counter” (CC), which is a circuit capable of counting all of the charge in the highly complex, high frequency current load waveforms encountered in mobile devices. When the battery is either fully discharged or fully charged, the CC is initialized accordingly. Knowledge of battery capacity coupled with the CC value is used to provide SOC information.
Some systems do not employ CC's for reasons of cost or unavailability at the time of design. Instead, less accurate techniques for determining SOC during the discharge phase of operation are employed. For example, battery resistance over load current, temperature, and age is characterized and recorded, and a table of SOC is created as a function of open circuit battery voltage (OCBV). During discharge, average voltage, average current, and temperature are measured and used along with the battery characteristics to calculate an equivalent OCBV. An SOC is then derived from the OCBV look-up table. In another example, all loads in a computer are enumerated and their magnitudes measured and recorded. A counter is incremented during the activation of each load and a crude, open loop CC is thereby implemented. The CC is reinitialized whenever the battery is fully charged.
However, the above discussed techniques are not well suited to determine SOC when the battery is being charged. During the final portion of the charge cycle, the battery voltage is held constant as the current falls from a constant value that was held during the first portion of the charge cycle to a lower, termination current level. As would be understood by those skilled in the art, when the voltage is constant, it affords no SOC information using the first method described above. With the second method, the inaccuracy of the SOC estimate at the last instance of discharge leads to problems even if a CC were suddenly made available to integrate charge current. With no means available to correct the original SOC error, the reported SOC could easily over or undershoot the actual SOC. As a result, the calculated SOC could reach 100% before the battery finishes charging (leading the user to remove the battery prematurely) or the battery could reach full charge but still indicate an SOC of under 100% (leading to wasted time).